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- Title
Van Allen Probes Observations of Oxygen Ion Cyclotron Harmonic Waves: Statistical Study.
- Authors
Wang, Yan; Liu, Kaijun; Min, Kyungguk; Yao, Fei; Xiong, Ying; Cheng, Kun; Liu, Yuqi; Zheng, Xianming; Zhou, Jingyi
- Abstract
We report the first statistical survey of the oxygen ion cyclotron harmonic waves observed by Van Allen Probes throughout the mission. Our statistical study shows that the waves occurred over 2 < L < 6 and across all magnetic local time. More than 50% of the events were observed just outside the plasmapause, and the typical wave amplitude is between ∼0.1 and several nT. The frequency spacing between two consecutive wave harmonics decreases with increasing L but stabilizes when L > 5. The frequency spacing is larger than the local oxygen ion gyrofrequency for many events, especially those observed at larger L, suggesting that these waves propagated there from lower L shell regions. The waves are mainly on the dayside at L > 4 under quiet geomagnetic conditions, but can occur at lower L shells with a more uniform MLT distribution under more active geomagnetic conditions. Plain Language Summary: As their name suggests, oxygen ion cyclotron harmonic (OCH) waves have discrete spectral peaks near multiple harmonics of the oxygen ion cyclotron frequency. These waves have been observed in the terrestrial magnetosphere but have not been thoroughly studied. Their excitation mechanism and propagation direction are still under debate. In this study, we perform the first statistical survey of these waves based on Van Allen Probes observations. An example event first demonstrates the general properties of typical OCH waves. The corresponding ion distribution suggests that the waves are excited by energetic oxygen ions with a ring‐like partial shell velocity distribution and can lead to the transverse heating of helium and oxygen ions less than a few hundred eV. Our statistical results show that the waves occurred in a large area while their spatial distribution varies with the geomagnetic activity level. Interestingly, the frequency spacing between two consecutive discrete spectral peaks tends to be smaller for the waves observed further away from the Earth and is mostly larger than the local oxygen ion gyrofrequency. Thus, many of the waves are probably not locally generated but have propagated there from lower altitudes. Key Points: We report the first statistical study of the oxygen ion cyclotron harmonic waves observed by Van Allen Probes throughout the missionThe waves occurred over 2 < L < 6 and across all magnetic local time. Over 50% of the events were observed just outside the plasmapauseThe frequency spacing between wave harmonics decreases with increasing L and is mostly larger than the local oxygen ion gyrofrequency
- Subjects
HELIUM ions; OXYGEN; IONS; DISTRIBUTION (Probability theory); CYCLOTRONS; MAGNETOSPHERE
- Publication
Geophysical Research Letters, 2022, Vol 49, Issue 4, p1
- ISSN
0094-8276
- Publication type
Article
- DOI
10.1029/2021GL096825